A highly transmissible strain of avian flu has killed tens of thousands of wild birds in the UK, but a Scottish seabird colony has revealed possible resistance to the infection.

The striking pale-blue eyes of some northern gannets became stark black in the globally-significant Bass Rock colony off the coast of North Berwick last year.

A study, conducted by the RSPB, has revealed that the discolouration of the iris is linked to previous infection with highly pathogenic avian influenza (HPAI) H5N1.

Experts hope that the discovery could prove to be a “non-invasive diagnostic tool” after the colony saw adult survival rates between 2021 and 2022 drop by 42% compared to the previous 10-year average.

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Bass Rock, which is the largest gannet colony in the world, was at the centre of a detailed study to understand the impact of the virus – and crucially proved that the seabird can recover from infection.

Lead author of the study Dr Jude Lane called it a “fascinating development” and added: “The next steps are to understand its efficacy, if it applies to any other species and whether there are any detrimental impacts to the birds’ vision.

“Ophthalmology exams will also be needed to determine what is causing the black colouration.”

Blood samples from 18 apparently healthy adult gannets with both normal and black irises were tested for bird flu antibodies. Eight tested positive, of which seven had black irises.

While avian flu has impacted birds for decades, the latest strain has seen changes in the outbreaks and the species affected.

Northern gannets appeared especially severely impacted last year, with unusually high mortality in all but one of 41 monitored colonies around the globe.

It will remain unclear just how many birds died at Bass Rock until they return for their breeding season.

The Scottish Seabird Centre experts, who monitor and maintain the colonies off the coast of North Berwick, observed “several variations in the extent” of the birds’ eye blackening.

Chief executive Susan Davies said: “The good news is that some of these infected birds have survived, returning to the Bass Rock for this year’s breeding season, and that suggests there is some resistance to this disease.”

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She added: “There has been a lot of scientific interest in many aspects of the 2022 outbreak of highly pathogenic avian influenza in Scotland’s internationally important seabird colonies.

“It was good to see more collaboration with the scientists who can help us gain a better understanding of the disease and its impact.

“This RSPB-led research from the Bass Rock provided confirmation of the early observations made by our team that infected Northern gannets’ eyes appeared to change from striking blue to jet black, with several variations of the extent of the blackening.”

However, despite the possible resistance to the infection, experts continue to “keep a watchful eye” as seabirds flock back to Scotland for their breeding season.

Seabirds are amongst the most threatened group of birds - out of the 25 breeding species, 24 are Red or Amber on the UK list of Birds of Conservation Concern.

The Scottish Seabird Centre, in partnership with the University of Edinburgh’s Airborne Research and Innovation Team, plan to fly two drone surveys in May and June.

Ms Davies added: “There is still a lot more to understand about the disease and its impact on the colony dynamics, breeding success and overall colony size.”

Once the drone surveys are analysed the centre will be able to “provide a colony level assessment of the number of occupied nest sites and total number of breeding Northern gannets that remain on the Bass Rock”.

However, the centre has relaunched their landing trips after they were halted to limit the transmission of bird flu among the colony.

“We still don’t know what will unfold this breeding season and will continue to keep a watchful eye on the colony,” she added.

“We are however delighted that we have been able to re-start the private Bass Rock photography trips which allow people direct access, under the steer of our experienced guide, into the colony.

“These gannet landing trips offer one of the most amazing, sense-busting experiences you can have.”